void main() {
  Person p = new Person();
  p.name = '张三';
  p.age = 20;
  p.gender = '男';
  p.address = '北京';
  p.say();
}

class Person {
  String? name;
  int? age;
  String? gender;
  String? address;

  say() {
    print('我叫$name,我今年$age岁,我是一个$gender,我住在$address');
  }
}
